Recent increases in demand for low-cost color Liquid Crystal Displays (LCDs) (for example, for use in portable “laptop” personal computers) have driven the need to develop increasingly more cost-effective LCD production methods. Color LCDs generally include a color filter array including individual colored filters (e.g., red (R), green (G) and blue (B)) that are formed over a liquid crystal material. During operation, the liquid crystal material is controlled to selectively pass or block light from passing through the color filter array in a way that creates a desired image.
Early conventional three-step processes for producing LCD color filters required separate processing steps for each of the three R, G and B color filters. For example, a dye-based process involved photolithographically patterning a water-soluble polymer material on a glass substrate, and then dipping the substrate in a dye bath to obtain a mono-colored (e.g., R) pattern. The patterning and dipping process is repeated three times to form the desired R, G and B color filter pattern. As another example, a pigment-based process involved patterning a photosensitive resin layer containing a color pigment on a substrate, and repeating for each of the three colors R, G and B. Problems with these conventional three-step processes include (a) the need to repeat essentially the same process three times is inherently more expensive than methods requiring a single processing step to produce all three color filters, and (b) an alignment defect occurring during any of the three repeated processes requires that the entire color filter array be scrapped, thus reducing production yields over single-step solutions.
A more recently proposed method for producing LCD color filters that addresses the problems associated with multi-step processes involves utilizing an ink jet system to “print” R, G and B color filter ink material in a single pass onto predetermined regions on a glass substrate. In this method, a black matrix (BM) material (e.g., polyimid) is deposited on a glass sheet and patterned to define an array of wells or basins separated by raised walls. The resulting BM glass structure is then passed under an ink jet head, which is controlled using known techniques to print (eject) small quantities (drops) of the color filter ink (i.e., color filter material dispersed in a solvent) into each of the wells. In theory, the raised walls surrounding each well serve to contain the color filter ink printed into that well until the ink has dried, thereby preventing intermixing of the different colored ink materials. After the color filter ink dries, the residual color filter material in each of the wells forms the desired RGB color filter array.
A problem with the ink jet approach arises because a relatively large amount of color filter ink is needed to produce a suitable color filter. After the color filter ink is printed (i.e., ejected from the ink jet head) into a well, the solvent evaporates from the color filter ink, leaving solid color filter material in the well. Due to the relatively large amount of solvent disposed in a given volume of color filter ink, and because the required thickness of the color filter film is comparable to the thickness of the BM walls, the volume of color filter ink entirely fills each well such that the ink extends above an upper surface of the BM walls, which can result in intermixing with adjacent ink quantities before the drying process is complete.
What is needed is an efficient method for producing color filter arrays for LCDs using ink jet technology that addresses the problems set forth above. In particular, what is needed is an efficient method for producing BM glass structures such that the color filter ink wets to the bottom surface and side walls of each well, and de-wets from the top surface of each well, thereby causing the color filter ink to reliably form a bead within the well, thereby forming the desired color filter structure when the ink dries.